This invention is directed to a metering device for use in implantable drug delivery systems. In particular, this invention is a fluid metering component that is inherently fail-safe yet utilizing very low operating power.
Implantable infusion systems are used in a number of medical applications. Typical are the INFUSAID Model 100 and Model 400 devices. Those systems are based on the technology embodied in U.S. Pat. No. 3,731,681, which employs a bellows drug reservoir that is driven by the use of a propellent in the form of a fluid liquid/vapor component such as Freon 11 (DuPont tradename). The liquid/vapor equilibrium is employed to pressurize the bellows drug reservoir at a positive pressure. These systems serially connect the drug containing reservoir to a capillary flow channel that meters the drug via viscous dissipation. The capillary flow employs a tube as a catheter at the exit, the situs of drug delivery. For a specific drug concentration, the flow rate is maintained at a relatively constant level depending upon the pressure difference between the reservoir and the catheter exit.
In more contemporary applications there is a necessity for the patient to vary the pump flow rate on a routine basis between pump refills. Examples of such therapies are: the continuous administration of insulin to counter diabetes, the bolusing of morphine for patient controlled analgesia and the alternate delivery and rest cycles used in chemotherapy of the liver. To provide such programmability, a number of different metering devices have been combined with positive pressure reservoirs. An example is the two-position solenoid used in conjunction with a volume accumulator as disclosed in U.S. Pat. No. 4,221,219. The system of the '219 patent allows different drug flows as a function of solenoid position by gating fluid into and out of the accumulator.
A self regulating flow restrictor having an adjustable set point is disclosed in U.S. Pat. No. 4,447,224. A system employing a leaking check valve in combination with a high pressure solenoid pump is disclosed in U.S. Pat. No. 4,714,462. A valve/accumulator/valve assembly employing a pressurized accumulator is disclosed in U.S. Pat. No. 4,838,887.
All of these prior art systems employ a positive pressure reservoir. That is, the reservoir is pressurized at a level higher than that of the outlet. This system configuration has both advantages and disadvantages as a function of the type of device used to meter the output. A positive pressure reservoir, for example, is advantageous in that it prevents large scale outgassing of drug solutions. This precludes the formation of air bubbles which would potentially alter the metering system dosage rate or provide danger to the patient by direct infusion of air. Additionally, the use of a positive pressure reservoir will significantly reduce the amount of energy required to meter flow, because less energy is needed in the controlled gating of a volume of fluid under pressure than in the active pumping of the same volume of fluid at the same pressure. However, inherent in the use of positive pressure reservoir technology is the potential for catastrophic flow in the event of a metering system failure. Unrestrained leak paths can potentially permit uncontrolled discharge of the reservoir contents. This may result in injury or death to the patient.
The valve/accumulator/valve system as typified in U.S. Pat. No. 4,838,887 has been particularly successful in achieving high accuracy of dosage delivery, relative immunity to entrapped air, a wide programmability range and safe operation. Moreover, this configuration requires moderate operating energies. This results in a reasonably long implant life for the system. However, inherently, these devices have a high initial cost associated with the sophisticated design, manufacture and assembly of discrete valve and accumulator components. These cost implications may prevent this pump configuration from achieving wide spread and common use.
An important aspect of the valve/accumulator/valve design is that the use of two valving elements provides the necessary redundancy to increase pump safety. The independent nature of the valves allows a mutually exclusive electronic "lock-out" in the event of a single point valve failure.
These first and second generation implantable pump configurations provided a baseline upon which the technology could be assessed and discrete improvements considered. There still exists an important need for a system which would have a lower energy requirement, thus permitting longer times between explant for replacement. Moreover, a follow-on design should have a smaller size than the original devices to permit implantation in a variety of different sites and a much lower cost of manufacture to increase the number of patients who could purchase the device and benefit from its use. Although the reliability, of the valve/accumulator/valve technology in particular has been shown to be exceptional, the use of two independent valves is not failsafe from a mechanical standpoint and could permit undetectable leakage modes. Therefore, new technology should be failsafe from a mechanical standpoint, for instance, providing for mutual exclusivity of valve position.